BMW Luxury Touring Community banner

1 - 8 of 8 Posts

·
Registered
Joined
·
323 Posts
Discussion Starter · #1 ·
This whole subject of slave cylinder housing drain hole procedure has been on my mind ever since I opened up my LT to perform the procedure. I have drilled the hole as per information off this site, paying particular attention to David Sheley's comments and Kevin Cooks detailed procedure.

My problem arises from the thought that this ia an early brake fluid/trans oil leak detection measure and relies on making the necessary repairs quickly before more damage is done. This is certainly better than leaving things to break down as BMW has set things up, but only gains some time in the end and maybe not much of that depending on the extent of the leak or leaks. The slave cylinder leak will show up much quicker by virtue of the clutch lever operation and will demand almost immediate attention. The transmission seal leak maybe not so for some time after the start of the leak.

I stand to be corrected, but, it is my understanding that the exposure of any oil based product on the 'rubber/s' of the slave cylinder and seal will quickly destroy those 'rubbers' and the reverse is true for brake fluid on the 'rubber' of the transmission seal. If this is correct it seems BMW has designed in a high potential major failure point in the bike. Who'd a thunk?

As I said, this whole situation has been bouncing around in my head lately, and I am not comfortable with reassembling it with just the hole drilled because I feel it is still a high potential for failure with maybe a little more time before major damage is done.

It seems to me it might help if there was some kind of barrier between the two seals. I don't know if this is possible or what it should be. My thinking is telling me it is more important to keep brake fluid out of the equation more so than transmission fluid because the brake fluid has the potential to contaminate the clutch causing unnecessary and costly repairs. I am of the mind the most likely cause of failure of the slave cylinder 'rubber' is contamination by transmission fluid.

Because of my experience with automotive brakes repair, I am of the mind an effective barrier might be a moly paste such as the Locktite Moly paste. If the nose of the clave cylinder, including the seal, was completely coated with the paste it may provide an effective barrier.

There, I have set myself up as a target, any and all shots are welcome, It's just that if this is way off base and a fatal shot is the only answer, be kind, make it a head shot.
 

·
Registered
Joined
·
12,165 Posts
Not much one can do to keep brake fluid out of the clutch other than the drain hole. Even if you could seal the nose of the cylinder to the bore, the brake fluid comes from inside the cylinder, around the piston when the seal fails, so it would just get funneled nicely by your paste more rapidly into the clutch. At least there is a little "reservoir" for leaking fluid around the nose of the cylinder, putting any other substance there would reduce the leak retaining volume, so would decrease the time before clutch failure.

If the rear transmission input shaft seal, right in front of the slave leaks, transmission fluid is eventually pushed forward into the clutch also. This is normally a slow leak though, so can exist for quite some time before enough collects to be pumped forward by the action of the clutch slave cylinder piston. The failing of the slave cylinder however will push brake fluid out at a higher rate, so the drain hole has to be large enough to allow this out instead of being forced into the clutch.
 

·
Registered
Joined
·
323 Posts
Discussion Starter · #4 ·
dshealey said:
Not much one can do to keep brake fluid out of the clutch other than the drain hole. Even if you could seal the nose of the cylinder to the bore, the brake fluid comes from inside the cylinder, around the piston when the seal fails, so it would just get funneled nicely by your paste more rapidly into the clutch. At least there is a little "reservoir" for leaking fluid around the nose of the cylinder, putting any other substance there would reduce the leak retaining volume, so would decrease the time before clutch failure.

If the rear transmission input shaft seal, right in front of the slave leaks, transmission fluid is eventually pushed forward into the clutch also. This is normally a slow leak though, so can exist for quite some time before enough collects to be pumped forward by the action of the clutch slave cylinder piston. The failing of the slave cylinder however will push brake fluid out at a higher rate, so the drain hole has to be large enough to allow this out instead of being forced into the clutch.
I am of the mind the most likely cause of failure of the slave cylinder will be because of an intrusion of trans. fluid causing damage to the slave cylinder seal 'rubber' and possibly the slave cylinder piston 'rubbers' as well. This in turn will cause brake fluid to be injected under pressure, from the operation of the clutch lever, into the area of the clutch rod and eventually into the clutch.

The reason I am thinking about coating the nose of the slave cylinder is to help keep any trans. fluid leak from directly contacting the slave cylinder seal.
If this can be done, it will help to prevent the failure of the slave cylinder seal from the outside. Thereby eliminating that as a cause for failure of the slave cylinder.

I was concerned that there might not be enough room around the end of the slave cylinder to permit the trans. fluid to drain out the drain hole if the nose of the cylinder is coated. By my measurements, there is about .094" between the end of the cylinder and the end of the machine bore in the housing. There is an additional larger space from the end of the machine bore to the face of the trans. seal that would provide more room for the leaking trans. fluid to accumulate as it migrates to the drain hole.

I agree, most trans. leaks will tend to be slow and the drain hole should be adequate, and provide an early enough warning. The slave seal 'protection' barrier should give more time particularly if there is a substantial trans. fluid leak.

One other thing. I am not sure if the moly paste is the appropriate barrier. A dielectric paste may be suited to that application. I have some brake system 'rubbers. I am going to experiment on and see if either is ok.
 

·
Registered
Joined
·
12,165 Posts
ajs said:
I am of the mind the most likely cause of failure of the slave cylinder will be because of an intrusion of trans. fluid causing damage to the slave cylinder seal 'rubber' and possibly the slave cylinder piston 'rubbers' as well. This in turn will cause brake fluid to be injected under pressure, from the operation of the clutch lever, into the area of the clutch rod and eventually into the clutch.

The reason I am thinking about coating the nose of the slave cylinder is to help keep any trans. fluid leak from directly contacting the slave cylinder seal.
If this can be done, it will help to prevent the failure of the slave cylinder seal from the outside. Thereby eliminating that as a cause for failure of the slave cylinder.

I was concerned that there might not be enough room around the end of the slave cylinder to permit the trans. fluid to drain out the drain hole if the nose of the cylinder is coated. By my measurements, there is about .094" between the end of the cylinder and the end of the machine bore in the housing. There is an additional larger space from the end of the machine bore to the face of the trans. seal that would provide more room for the leaking trans. fluid to accumulate as it migrates to the drain hole.

I agree, most trans. leaks will tend to be slow and the drain hole should be adequate, and provide an early enough warning. The slave seal 'protection' barrier should give more time particularly if there is a substantial trans. fluid leak.

One other thing. I am not sure if the moly paste is the appropriate barrier. A dielectric paste may be suited to that application. I have some brake system 'rubbers. I am going to experiment on and see if either is ok.
No paste or gel will stay in place. Remember, the piston moves back and forth as the clutch lever is pulled in and released. That would very effectively destroy any sealing you may acheive on initial application.

I posted pictures once of one of my failed slave cylinders, and it was very apparent that the bearing in the end of the little piston had tightened/siezed causing the piston to spin with the actuation rod, wiping out the piston and bore of the cylinder badly. No evidence of a transmission oil leak at the same time though.

That is a very small "throwout" bearing in the end of the piston, which takes all the force of releasing the clutch, and it is spinning anytime the engine is running. Not a very robust design at all. Sure expects a lot out of such a small thrust bearing, with only the little amount of grease in it applied at the factory. Mine came apart when I disassembled it, only a few very small balls to do all the work.
 

·
Registered
Joined
·
323 Posts
Discussion Starter · #6 ·
That is a very small "throwout" bearing in the end of the piston, which takes all the force of releasing the clutch, and it is spinning anytime the engine is running. Not a very robust design at all. Sure expects a lot out of such a small thrust bearing, with only the little amount of grease in it applied at the factory. Mine came apart when I disassembled it, only a few very small balls to do all the work.[/QUOTE]

Thanks for this information. All my thinking about this subject has been on the basis that the clutch push rod was set in a cup on the end of the slave cylinder piston as is the case with any other hydraulic clutch system I am familiar with. This, in IMO, is the real weak point in the design of this system and means it is likely to be the most probable cause of failure. I can see, with the clutch rod spinning at engine speed there would be a tremendous load on the 'throwout' bearing and presents a high potential to inject brake fluid under pressure into the area surrounding the clutch rod. I can now understand the point you make about not maintaining an effective seal for the nose area of the slave cylinder.

It would be more important to protect that area from an incursion of brake fluid than trans. fluid. If that is even possible. One question, Mr. Shealey; Does the clutch rod spin at engine speed at all times, eg even when the clutch is disengaged, or only when it is engaged?

Thanks again for the new (to me anyway) information. If I cannot improve the situation past drilling the hole, I will at least be happy that all has been done that could be done and get on with the job.
 

·
Registered
Joined
·
2,193 Posts
ajs said:
Thanks for this information. All my thinking about this subject has been on the basis that the clutch push rod was set in a cup on the end of the slave cylinder piston as is the case with any other hydraulic clutch system I am familiar with. This, in IMO, is the real weak point in the design of this system and means it is likely to be the most probable cause of failure. I can see, with the clutch rod spinning at engine speed there would be a tremendous load on the 'throwout' bearing and presents a high potential to inject brake fluid under pressure into the area surrounding the clutch rod. I can now understand the point you make about not maintaining an effective seal for the nose area of the slave cylinder.
My experience with two failures has indeed been the rubber boot. In dissecting both of them it appears that over time the rubber boot sheds very small balls of material that eventually provide a path (think Hansel and Gretel) along the rod for flow of the brake fluid. I have obtained seals of another material from a friend who does hydraulic engineering. It was as cheap to buy five as it was fifty so if you want a couple send me a pm and I'll send them on to you. I have not put one of my rebuilt units on the bike yet as I have not had an occassion to pull the swing arm or encounter another failure.

JM2C and worth no more
 

·
Registered
Joined
·
127 Posts
I have yet to hear of anyone claiming a "save" as a result of drilling the weep hole??
 
1 - 8 of 8 Posts
Top